Many manufactured articles require surface properties that differ from the bulk properties of the material from which the article is made. Some products, such as automotive engine block castings have cylinder surface regions that experience continual frictional contact by reciprocating pistons and require wear resistance of a degree not inherent in the porous surface of the casting. Other articles could be made of lighter weight or lower cost materials, or by a lower cost manufacturing process, if the microstructure of a surface (such as a porous surface) could be altered to provide it with properties more preferred than the bulk properties of the material resulting from the manufacture of the article.
Friction stir welding is a solid-state welding process used, for example, to form a weld seam between abutting metal workpieces. A rotating tool with a profiled tip and of suitably hard material makes contact with the workpieces and is pushed or plunged into the joint region. The friction of the impinging and rotating contact heats and plasticizes the abutting regions of the metal workpieces. The metal is heated to a suitable depth for the weld, often substantially through the thickness of the abutting pieces. Plasticized material flows around the tool and coalesces behind the rotating tool as it is moved along the intended weld seam. Since only the joint area is heated by the moving and rotating tool, the flowing weld metal is re-hardened by heat loss to the adjacent unheated workpiece material.
It is now realized that a rotating tool like that used in friction stir welding could be utilized to hot work or otherwise thermally process a surface layer of an article to obtain desired physical and/or chemical properties in the selectively treated area. In other words, the rotating tool could be used in friction stir thermal processing of a selected surface area of an article.